This invention relates to an imaging system employing an array of radiation detectors and, more particularly, the use of a Fresnel pattern impressed upon signals received by the detectors to provide for imaging as in the case of the sonic imaging of a human subject.
Fresnel masking has been utilized in both electromagnetic and sonic imaging systems. With respect to electromagnetic imaging systems, a Fresnel pattern is disclosed in the U.S. Pat. No. 3,263,079 which issued to L. N. Mertz and N. O. Young on July 26, 1966 wherein the pattern is utilized for forming the image of stars in the sky. The use of a Fresnel pattern in nuclear medicine for forming an image of a radioactive source is disclosed in the U.S. Pat. No. 3,936,639 which issued in the name of H. H. Barrett on Feb. 3, 1976. The use of a Fresnel pattern impressed upon the signals of sonic radiation detectors is disclosed in the U.S. Pat. No. 3,911,730 which issued in the name of L. Niklas on Oct. 14, 1975 wherein the energization of groups of radiation detectors, or transducers, is employed following the arrangement of a Fresnel pattern in at least one dimension. The use of an ultrasonic imaging scanner for imaging organs of the human body is disclosed in the U.S. Pat. No. 3,805,596 which issued in the name of C. N. Klahr on Apr. 23, 1974.
The use of the Fresnel pattern for sonic imaging systems is advantageous in that the Fresnel pattern provides for the focussing of the sonic radiation in the manner of a lens. A problem arises in that with systems of the prior art, the Fresnel pattern, whether it be utilized with a one dimensional line array or in a two dimensional array, produces the effect of both a converging pattern of radiation which converges toward a focal point in the subject in front of the array as well as a diverging radiation pattern which emanates from a virtual focus located behind the array. The energy content of signals produced by the transducers in response to incident sonic energy from the diverging radiation pattern approximately equals that of the energy content of signals associated with the desired converging pattern. As a result, there is substantial unwanted noise which degrades an image of the subject obtained with the converging radiation pattern.